The renal response to osmotic diuretics is probably due to the interplay of several factors. The primary effect involves an increased fluid loss caused by the osmotically active diuretic molecules; this results in reduced Na+ and water reabsorption from the proximal tubule.
An additional contributing factor to the diuresis induced by osmotic diuretics is the increase in renal medullary blood flow that follows their administration. This medullary hyperemia reduces the cortex-medullary osmolar gradient by carrying away interstitial Na+ and urea. This partial reduction of the osmolar gradient impairs normal reabsorption of tubular water, which occurs from the descending limb of Henle and the collecting duct.
Finally, there is an additional increase in electrolyte excretion due to impairment of ascending limb and distal tubule Na+ reabsorption; this occurs as a result of lowered tubular Na+ concentration and the increased tubular fluid flow rate.
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